Completed furnace, maintenance and inspection. I cleaned flame sensor and wiped down furnace. I tested air pressure and temp splits. I tested components and amp draws. I ran a combustion analyzer test and carbon monoxide test. As of today, the 15 year-old furnace is running safely however there are multiple signs of the heat exchanger giving out. I showed customer the safety concerns. I recommended to get low level detection carbon monoxide detectors. Customer would like to wait on taking care of the furnace situation for now. I offered our membership to continue maintenance on the furnace, AC, and water heater. Furnace is operational at departure.